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Introduction to Linux - A Hands on Guide

This guide was created as an overview of the Linux Operating System, geared toward new users as an exploration tour and getting started guide, with exercises at the end of each chapter.
For more advanced trainees it can be a desktop reference, and a collection of the base knowledge needed to proceed with system and network administration. This book contains many real life examples derived from the author's experience as a Linux system and network administrator, trainer and consultant. They hope these examples will help you to get a better understanding of the Linux system and that you feel encouraged to try out things on your own.

Some command interpreters maintain a hash table of frequently used commands.

This eliminates having to search through a long PATH environment variable to find a corresponding executable for a command.

It can cause some problems when the command is added to a directory in the path, but not where the hash table has it recorded. These interpreters also include a "rehash" command that causes it to make a pass through the directories and identify new entries, and generate a new hash table.

The "type" command allows you to identify what is actually going to be used when a given command is issued. If a command is located in the hash table it lets you know that (useful if you expected a different result in a command).

The way shells interpret commands is also why the "." is normally excluded from the search path as a security hazard. You really don't want someone elses command to be picked when you really wanted the one you always use...